The invention relates to polyurethane molding compositions, such as polyurethane casting compositions, which are prepared by the polyaddition of diisocyanates to polyols or from prepolymers with appropriate end groups. More specifically, the invention relates to polyurethane molding compositions containing catalysts which aid such compositions in passing from the liquid to the solid state and in setting in economically justifiable times.
Polyurethane molding compositions, such as casting compositions, are utilized in medical technology, for example, in dialysis and diafiltration equipment for the treatment of blood and blood fluids. Since these blood and blood fluids are returned to the blood flow after dialysis or diafiltration, it is necessary that all components of casting compositions, which come into contact with the blood or blood fluids, are absolutely safe physiologically and completely nonpoisonous.
For this reason, polyurethane molding compositions, when used in medical applications, cannot be hardened with many well-known catalysts, since these catalysts either must be avoided for toxicological reasons and/or have other disadvantages. Accordingly, organic tin compounds and tertiary amines are not absolutely safe for toxicological reasons. Moreover, the tertiary amines also have the disadvantage that they lead to the formation of bubbles in many systems, especially if the work is not conducted with the complete exclusion of moisture.
Other well-known, especially salt-like catalysts, have only limited solubility in the polyol component and, being a solid substance, are not readily measured out. When dissolved in the diiscyonate component, they lead to undesirable side effects, and decomposition and resinification are observed on storage.
For toxicological reasons, acids, such as hydrochloric acid, acetic acid and trichloroacetic acid, also may not be used in medical technology, for example, when casting hollow-fiber or flat-film dialyzers.
For this reason, a catalyst-free polyurethane molding composition is suggested by German Offenlegungsschrift No. 2,340,661, so that this molding composition contains no toxicologically doubtful components. However, such casting compositions have a disadvantage in that they require a relatively long period of setting, that the setting must be carried out at comparatively high temperatures and that the catalyst-free setting can only be carried out with specific compositions of the molding material.
It is well known that certain physiologically absolutely safe carboxylic acids may be used as catalysts for polyurethane molding compositions. According to the U.S. Patent, oleic acid is used for this purpose, according to Chemical Abstracts, vol. 84, 1976, reference 61474p, octanoic acid is used, and according to the British Pat. No. 1,453,152, an alkyl or alkoxy-substituted benzoic acid is used. Such catalysts, however, give relatively long mold-release times, which lead to a decreased output per day.
Thus, polyurethane molding compositions and processes for making molding compositions which are not poisonous, which are physiologically absolutely safe, which do not give off any physiologically doubtful material on contact with blood or other body fluids, and which provide an accelerated mold-release time, i.e., to reach as quickly as possible a degree of hardness which will permit the molding to be taken from the mold without danger of deformation, are highly desirable.